In carrying out the above vacuum casting method, it is desirable to enhance the degree of vacuum in the cavity. With a higher degree of vacuum, it is possible to prevent generation of blowholes in the molten metal, and this permits obtaining high quality casting. In order to enhance the degree of vacuum in the cavity, it is required to enhance the sealing capacity of the gate for blocking the communication between the cavity and the molten metal passage. A casting apparatus to this end has been proposed in Japanese Laid-Open Utility Model Publication No. 3-31058. This will now be described with reference to FIG. 5.
As shown in FIG. 5, this apparatus comprises a cavity 202 formed in a casting die 201. The cavity 202 can communicate with a molten metal passage 210 via a sprue 202a. The molten metal passage 210 is immersed in molten metal 206 heated and melted in a melting furnace 208. A gate piston 212 is provided for switching on and off the communication between the cavity 202 and the molten metal passage 210. The gate piston 212 is lowered to bring its lower end surface 212c into abutting engagement with an abutment surface 215 formed on the casting die, thereby blocking the communication between the cavity 202 and the molten metal passage 210. A vacuum pump 204 is connected to the cavity 202. A pressure reducing pump 220 is connected to the molten metal passage 210 via a passage 212b. Designated at 212a is an exhaust vent.
This apparatus is operated as follows. First, the gate piston 212 is lowered to bring the lower end surface 212c into abutting engagement with an abutment surface 215. Thus, the communication between the cavity 202 and the molten metal passage 210 is blocked. In this state, the pressure in the cavity 202 is reduced by operating the vacuum pump 204. Then, the pressure reducing pump 220 is operated to produce a negative pressure in the molten metal passage 210, thus causing the molten metal 206 to be sucked up to the position right below the gate piston 212.
In this state, the piston lower end surface 212c should desirably be in fully tight contact with the abutment surface 215, but in view of machining accuracy, it is difficult to bring these surfaces into full surface contact. As a result, with the apparatus shown in FIG. 5, molten metal can enter into a minute clearance formed between the piston lower end surface 212c and the abutment surface 215.
When the molten metal enters into the minute clearance between the piston lower end surface 212c and the abutment surface 215, it becomes solidified, and the communication between the cavity 202 and the molten metal passage 210 is completely sealed. Thus, the apparatus shown in FIG. 5 can obtain a sufficiently high degree of vacuum in the cavity 202.